If you deprive an organism of stressors, you affect its epigenetics and gene expression—some genes are up-regulated (or down-regulated) by contact with the environment. A person who does not face stressors will not survive should he encounter them. Just consider what happens to someone’s strength after he spends a year in bed, or someone who grows up in a sterile environment and then one day takes the Tokyo subway, where riders are squeezed like sardines.
